Stationary turbo machines of comparable larger size predominantly use journal bearings of the above mentioned type to support rotor shafts. Stationary turbo machines are for example gas turbines, compressors or steam turbines. One example of a journal bearing according to the above mentioned type is given by EP 2 345 820 A1.
Journal oil bearings as defined incipiently are lubricated by oil and in most cases have an unlimited lifetime if design speed operation is considered. Design speed according to the invention is the speed of the rotor, for which the journal bearing was designed for respectively which speed was assumed in order to design the bearing. During design speed operation the journal surface of the rotor shaft is carried by an oil film and does not directly contact the bearing surface of the bearing. One disadvantage of the journal bearing is the dependency of the oil film on a minimum rotation speed—also considered as the lift-off-speed of the journal bearing. During operation of the journal bearing below the lift-off-speed significant wear occurs. Further the build-up of the oil film is significantly influenced by the surface structure of the bearing surface and its according adjustment to the journal surface. If tolerances of the geometry of the bearing surface don't meet the magnitude of micrometer bearing temperatures can exceed allowable limits, which might endanger safe operation.
In most aero gas turbines this problem is avoided by using ball bearings which need to be overhauled periodically.
To enable an effective build-up of the oil film, which carries the rotor by a certain pressure distribution in the oil film gap between the bearing surface and the journal surface, the pads are designed as tilting pads, enabling a tilting movement around an axis parallel to the rotor axis. An increase in rotation speed normally increases positive tilting of the tilting pad caused by a change in pressure distribution in the oil film. The wearing thickness of the oil film, the wearing angle position of the tilting pads depending on the operation speed leads to the disadvantage of a displacement of the rotor shaft axis with varying operation speed. This radial displacement becomes significantly large during lift-off of the rotor shaft since the oil film is established.
Next to these disadvantages relating to radial displacement by oil film variation another radial displacement mechanism of conventional journal bearings is caused by the tilting movement of the pads. Especially during start-up and run-down involving a standstill the tilting pads cause a shaft sink in the order of several tense of a millimeter respectively micrometer. These shafts think needs to be compensated by additional eccentric clearance between moving parts and stationary parts. The design of radial clearance of a turbo machine is significantly influenced by this shaft sink to avoid radial contact between the rotor and the stator.